The skin presents the largest, most readily accessible biological membrane through which an analyte may be collected or a drug delivered. Mucosal and buccal membranes present feasible, but less accessible, sites for collection and delivery. Unfortunately, the skin and, to a somewhat lesser extent, the mucosal and buccal membranes, are highly resistant to the transfer of materials therethrough. The skin generally comprises two main parts: the epidermis and the dermis. The epidermis forms the outer portion of the skin, and itself comprises several distinct layers. The outermost layer of the epidermis, the stratum corneum, is composed of denucleated, keratinized, clear, dead cells, and is typically between 10-30 microns thick.
The stratum corneum is chiefly responsible for the well known barrier properties of skin. Thus, it is this layer that presents the greatest barrier to transdermal flux of drugs or other molecules into the body and of analytes out of the body. The stratum corneum, the outer horny layer of the skin, is a complex structure of compact keratinized cells separated by intercellular lipid domains. Compared to the oral or gastric mucosa, the stratum corneum is much less permeable to molecules either external or internal to the body. The stratum corneum is formed from keratinocytes, which comprise the majority of epidermis cells, that lose their nuclei and become corneocytes. These dead cells then form the stratum corneum, which is a very resistant waterproof membrane that protects the body from invasion by exterior substances and the outward migration of fluids and dissolved molecules. The stratum corneum is continuously renewed by shedding of corneocytes during desquamination and the formation of new corneocytes by the keratinization process.
The formation of micropores (i.e. microporation) or delivery openings through the stratum corneum to enhance the delivery of drugs has been the subject of various studies and has resulted in the issuance of patents for such techniques.
Paranjape, et al., “A PDMS dermal patch for non-intrusive transermal glucose sensing,” (Sensors and Actuators, May 2003, 195-204) discloses a polydimethylsiloxane (PDMS) patch for performing controlled and non-invasive monitoring of glucose levels. The PDMS patch is used in conjunction with a microporation system to open micropores through the stratum corneum of a patient. The micropores are formed by ablating the skin tissue through the use of micro-heaters integrated on the side of the patch that contacts the skin. Monitoring of glucose levels is then achieved using the patch.
Tankovich, U.S. Pat. No. 5,165,418, discloses a method of obtaining a blood sample by irradiating human or animal skin with one or more laser pulses of sufficient energy to cause the vaporization of skin tissue so as to produce a hole in the skin extending through the epidermis and to sever at least one blood vessel, causing a quantity of blood to be expelled through the hole such that it can be collected. Tankovich '418 thus is inadequate for noninvasive or minimally invasive permeabilization of the stratum corneum such that a drug can be delivered to the body or an analyte from the body can be analyzed.
Tankovich et al., U.S. Pat. No. 5,423,803, discloses a method of laser removal of superficial epidermal skin cells in human skin for cosmetic applications. The method comprises applying a light-absorbing “contaminant” to the outer layers of the epidermis and forcing some of this contaminant into the intercellular spaces in the stratum corneum, and illuminating the infiltrated skin with pulses of laser light of sufficient intensity that the amount of energy absorbed by the contaminant will cause the contaminant to explode with sufficient energy to tear off some of the epidermal skin cells. Tankovich '803 further teaches that there should be high absorption of energy by the contaminant at the wavelength of the laser beam, that the laser beam must be a pulsed beam of less than 1 microsecond duration, that the contaminant must be forced into the upper layers of the epidermis, and that the contaminant must explode with sufficient energy to tear off epidermal cells upon absorption of the laser energy. This invention also fails to disclose or suggest a method of drug delivery or analyte collection.
Raven et al., WO 92/00106, describes a method of selectively removing unhealthy tissue from a body by administering to a selected tissue a compound that is highly absorbent of infrared radiation of wavelength 750-860 nm and irradiating the region with corresponding infrared radiation at a power sufficient to cause thermal vaporization of the tissue to which the compound was administered but insufficient to cause vaporization of tissue to which the compound had not been administered. The absorbent compound should be soluble in water or serum, such as indocyanine green, chlorophyll, porphyrins, heme-containing compounds, or compounds containing a polyene structure, and power levels are in the range of 50-1000 W/cm2 or even higher.
Konig et al., DD 259351, teaches a process for thermal treatment of tumor tissue that comprises depositing a medium in the tumor tissue that absorbs radiation in the red and/or near red infrared spectral region, and irradiating the infiltrated tissue with an appropriate wavelength of laser light. Absorbing media can include methylene blue, reduced porphyrin or its aggregates, and phthalocyanine blue. Methylene blue, which strongly absorbs at 600-700 nm, and a krypton laser emitting at 647 and 676 nm are exemplified. The power level should be at least 200 mW/cm2.
Early prototype microporation systems were successful in creating delivery openings in selected biological membranes, such as the skin, to allow the efficient delivery of permeant compounds into the subject's body. However, there still remains a need to quantify and more clearly describe optimal delivery openings in a biological membrane. More particularly, there exists a need to develop a method for consistently measuring the depth and morphology of the delivery opening in order to optimize the use of the microporation system in delivering therapeutically active substances and extracting analytes from the body to be analyzed.
While many of the early prototype microporation systems allow for delivery of permeant compounds across a biological membrane, the preferred mode of delivery for many of such compounds is still transcutaneously by way of an injection using a hollow needle coupled to a syringe. In other words, a large percentage of curent permeant agents are administered to a patient through the skin by a hypodermic needle, which punctures the skin and then delivers a liquid bolus of the drug formulation. There is also a need, therefore, for a method of transdermally delivering these sorts of permeant substances to a patient in need thereof wherein the serum concentration profile of the permeant in the body when delivered by the microporation system mimics that of a permeant delivered by way of a hypodermic needle.